Acoustic doppler system and method

ABSTRACT

A survey system including a multibeam echo sounder having a projector array and a hydrophone array in a Mills Cross arrangement uses a multicomponent message to ensonify one or more fans to estimate a Doppler velocity.

PRIORITY APPLICATION AND INCORPORATION BY REFERENCE

This application is a continuation of U.S. patent application Ser. No.15/807,406 filed Nov. 8, 2017 which is a continuation of U.S. patentapplication Ser. No. 15/495,802 filed Apr. 24, 2017 now U.S. Pat. No.9,817,116 which is a continuation-in-part of U.S. patent applicationSer. No. 15/476,137 filed Mar. 31, 2017 now U.S. Pat. No. 10,132,924which claims the benefit of U.S. Prov. Pat. App. No. 62/329,631 filedApr. 29, 2016 and this application claims the benefit of 62/423,055filed Nov. 16, 2016 all of which are included herein by reference, intheir entirety and for all purposes. This application incorporates byreference, in their entireties and for all purposes, the disclosures ofU.S. Pat. No. 3,144,631 concerning Mills Cross sonar, U.S. Pat. No.5,483,499 concerning Doppler frequency estimation, U.S. Pat. No.7,092,440 concerning spread spectrum communications techniques, U.S.Pat. No. 8,305,841 concerning sonar used for mapping seafloortopography, and U.S. Pat. No. 9,244,168 concerning frequency burstsonar.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to underwater acoustical systems, methodsfor using underwater acoustical systems, and methods for processing andusing the data they produce. In particular, the invention relates tosurvey systems including sonar systems capable of making Dopplermeasurements such as Doppler velocities associated with multi-fanensonification of fixed or moving targets.

Discussion of the Related Art

A month after the Titanic struck an iceberg in 1912, Englishmeteorologist Lewis Richardson filed a patent at the British PatentOffice for an underwater ranging device. Modern day successors toRichardson's invention are often referred to as SONAR (sound navigationand ranging) devices.

Modern day SONAR devices include ones capable of making Dopplermeasurements to determine velocities. Where there is relative motionbetween a target and a SONAR that ensonifies the target, echoes from thetarget may be used to determine relative target velocities. For example,where a SONAR moves relative to a fixed target, echoes from the targetmay be used to determine SONAR velocities.

Such Doppler velocity measurements are subject to multiple sources oferror. Such errors limit the utility of an otherwise useful survey andnavigation aid technology.

SUMMARY OF THE INVENTION

The present invention provides a multifan survey system and method.Multifan survey operations may be useful in multiple survey tasksincluding bathymetry, water column monitoring, forward look survey,Doppler velocimetry, Doppler current profiling, and motionstabilization.

Doppler velocimetry may benefit from multifan operation with advantagesincluding use of one or more of forward/backward steered fans that allowfor a Janus-like configuration of beams from a multi-beam echo sounder.Doppler estimates like Doppler velocity log (“DVL”) estimates may bemade. Doppler estimates like Acoustic Doppler Current Profiling (“ADCP”)estimates may be made.

In an embodiment, an acoustic Doppler system for estimating a relativevelocity between an acoustic source and reflectors and/or scatterers inmultiple underwater fans comprises: one or more transducers in one ormore projector arrays included in the acoustic source; a transmitter fortransmitting a message via the one or more projector arrays, the messagefor ensonifying i>=2 fans; components of the message including at leastone pulse pair for each fan and each fan including j>=8 beams; one ormore transducers of the one or more projector arrays and pluraltransducers of one or more hydrophone arrays in a Mills Crossarrangement; the hydrophones for sensing reflected and/or scatteredreturns and a receiver for processing reflected and/or scattered returnsignals; and, the return signals processed via autocorrelation of pulsepairs to calculate for each of (i*j) beams respective Doppler radialvelocity estimates DRV_(i,j); wherein simultaneous consideration of aplurality of the DRV_(i,j) estimates provides an estimated sourcevelocity.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is described with reference to the accompanyingfigures. These figures, incorporated herein and forming part of thespecification, illustrate embodiments of the invention and, togetherwith the description, further serve to explain its principles enabling aperson skilled in the relevant art to make and use the invention.

FIG. 1A shows a survey system including a multibeam echo sounder systemof the present invention.

FIGS. 1B-F show embodiments of at least portions of the multi-beam echosounder system of FIG. 1A.

FIG. 1G shows a legend of selected symbols.

FIGS. 2A-B show message cycles for use with the multibeam echo soundersystem of FIG. 1A.

FIGS. 3A-F show multifan survey system operations for use with themultibeam echo sounder system of FIG. 1A.

FIGS. 4A-I show source velocity evaluation calculations and methods foruse with the multibeam echo sounder systems similar to that of FIG. 1A.

FIG. 4J shows velocity calculations and methods for use with multibeamecho sounder systems similar to that of FIG. 1A where the operating modeis a Doppler Velocity Log (“DVL”) mode.

FIG. 4K shows velocity calculations and methods for use with multibeamecho sounder systems similar to that of FIG. 1A where the operating modeis an Acoustic Doppler Current Profiler (“ADCP”) mode.

FIGS. 5A-F show pulse-pair messages for use with the multibeam echosounder system of FIG. 1A.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The disclosure provided in the following pages describes examples ofsome embodiments of the invention. The designs, figures, and descriptionare non-limiting examples of the embodiments they disclose. For example,other embodiments of the disclosed device and/or method may or may notinclude the features described herein. Moreover, described features,advantages or benefits may apply to only certain embodiments of theinvention and should not be used to limit the disclosed invention.

As used herein, the term “coupled” includes direct and indirectconnections. Moreover, where first and second devices are coupled,intervening devices including active devices may be locatedtherebetween.

Multibeam Echo Sounder

FIGS. 1A-E show a survey system including a multibeam echo soundersystem and describe exemplary multibeam echo sounder embodiments. FIG.1G shows a legend 100G of selected symbols appearing on FIGS. 1C-F.

FIG. 1A shows a survey system in accordance with an embodiment of thepresent invention 100A. The survey system includes an echo soundersystem such as a multibeam echo sounder system 102 which may be mountedon a surface vehicle or vessel, on a waterbody bottom, on a remotelyoperated vehicle, on an autonomous underwater vehicle, or the like. Asis further described below, echo sounder and/or survey system outputs114 may be contemporaneous with echo sounder processing of hydrophonedata as in some embodiments for bathymetry or non-contemporaneous withprocessing of hydrophone data as in some embodiments for waterbodybottom classification.

Data acquired by multibeam echo sounder systems 104 include data fromecho sounder listening devices such as hydrophones (e.g., transducers)that receive echoes which are related to the acoustic/pressure wavesemanating from the echo sounder projectors but have returned by virtueof an interaction with inhomogeneities of many kinds. The interactionsmay take the form of reflection or scattering. The inhomogeneities, alsoknown as reflectors and scattering centers, represent discontinuities inthe physical properties of the medium. Exemplary scattering centers maybe found in one or more of i) an ensonified volume of the waterbody suchas a water column, ii) upon the ensonified surface of the bottom, oriii) within the ensonified volume of the sub-bottom.

Scattering centers of a biological nature may be present in the watercolumn, as they are a part of the marine life. Scattering centers of anonbiological nature may be present in the water column in the form ofbubbles, dust and sand particles, thermal microstructure, and turbulenceof natural or human origin, such as ships' wakes. Scattering centers onthe surface of the bottom may be due to the mechanical roughness of thebottom, such as ripples, or be due to the inherent size, shape andphysical arrangement of the bottom constituents, such as mud, sand,shell fragments, cobbles and boulders, or due to both factors.Scattering centers in the sub-bottom may be due to bioturbation of thesediments, layering of different sediment materials within the bottom orburied manmade structures such as pipelines.

Data processing within the echo sounder system may includecontemporaneous processing of hydrophone data 106, for example to obtainDoppler velocity data, bathymetric data, and/or backscatter data. Dataprocessing may also include non-contemporaneous processing of multi-beamecho sounder system data 108, for example to characterize bottomconditions or the water column.

Data processing may include utilization of complementary or other data.For example, contemporaneous processing of hydrophone data 106 mayutilize contemporaneous 110 and/or non-contemporaneous 112 data such ascontemporaneously collected geographic positioning system (“GPS”) data,sound speed measurements, attitude, and navigational information. Forexample, non-contemporaneous processing of echo sounder system data mayutilize contemporaneous 110 and/or non-contemporaneous 112 data such asnon-contemporaneously collected waterbody bottom composition data andtidal records.

FIG. 1B shows portions of an exemplary multibeam echo sounder system(“MBES”) 100B. The echo sounder system includes a transducer section 120and an acoustic transceiver 122. The echo sounder system may include atransceiver interface such as an interface module 124 and/or aworkstation computer 126 for one or more of data processing, datastorage, and interfacing man and machine. Exemplary transducers, shownhere in a Mills Cross arrangement 120, include a transmitter orprojector array 130 and a receiver or hydrophone array 140. Projectorsin the projector array may be spaced along a line that is parallel witha keel line or track of a vehicle or vessel to which they are mountedwhich may be referred to as an along track arrangement. In someembodiments, a receiver of the transceiver 122 has an operatingfrequency range corresponding to that of the projectors and/or thehydrophones.

During echo sounder operation, sound or pressure waves emanating fromthe projector array travel within a body of water and possibly withinthe bottom beneath the body of water and in doing so may undergointeractions, such as reflections or scattering, which disturb thepropagation trajectory of the pressure waves. Some of the reflections orechoes are “heard” by the hydrophone array. See for example thedisclosure of Etal, U.S. Pat. No. 3,144,631, which is included herein byreference, in its entirety and for all purposes.

The acoustic transceiver 122 includes a transmitter section 150 and areceiver section 170. The acoustic transceiver may be configured totransmit to one or more projector arrays 130 and to receive from one ormore hydrophone arrays 140. Unless otherwise noted, the term transceiverdoes not require common packaging and/or encapsulation of thetransmitter and receiver.

In various embodiments, a projector array may be a single projectorarray regardless of the geometry, arrangement, or quantity of devicesemployed. For example, where a plurality of projectors forms a pluralityof spatially distinct projector groups, the plural projectors are asingle projector array if they are operated to ensonify the entirety ofa swath on a single ping, for example a swath of waterbody bottom or aswath of water column. In various embodiments: i) a single projectorarray may ensonify the entirety of a swath on a single ping; a pluralityof projector arrays may ensonify the entirety of a swath on a singleping; and, iii) a plurality of projector arrays ensonify multiple swathson a single ping.

The echo sounder may further include a means such as an interface module124 for interconnection with the transceiver 122. This interface modulemay provide, among other things, a power supply for the transceiver,communications with the transceiver, communications with the workstationcomputer 126, and communications with other sources of data such as asource of contemporaneous GPS data.

The workstation computer 126 may provide for one or more of dataprocessing such as data processing for visualization of survey results,for data storage such as storage of current profiling data, bathymetrydata, sound speed data, and backscatter data, for user inputs, and fordisplay of any of inputs, system status, and survey results.

FIG. 1C shows portions of an exemplary multibeam echo sounder system(“MBES”) 100C. The echo sounder system includes a transducer section120, a transmitter section 150, and a receiver section 170. Someembodiments include a sensor interface section 190 and/or a managementsection 192. And in some embodiments it is the management block thatsignals and/or provides instructions to the signal generators 159.

The transducer section includes transducers for transmitting acousticmessages and transducers for receiving acoustic messages. For example, atransducer section may include an array of projectors 130 and an arrayof hydrophones 140.

Projectors in the projector array 130 may include piezoelectric elementssuch as ceramic elements. Element geometries may include circular andnon-circular geometries such as rectangular geometries. Some projectorshave an operating frequency range of about 10 kHz to 100 kHz, of about50 kHz to 550 kHz, or about 100 to 1000 kHz.

Hydrophones in the hydrophone array 140 may include piezoelectricelements such as ceramic elements. Element geometries may includecircular and non-circular geometries such as rectangular geometries.Some hydrophones have an operating frequency range of about 10 kHz to100 kHz, of about 50 kHz to 550 kHz, or about 100 to 1000 kHz.

During operation of the projector array 130 and hydrophone array 140,the transmitter section excites the projector array, an outgoing message137 emanates from the projector array, travels in a liquid medium to areflector or scattering center 138, is reflected or scattered, afterwhich a return or incoming message 139 travels to the hydrophone array140 for processing by the receiver 170. Notably, the acoustic/pressurewave input 136 received at the hydrophone array 140 may include aDoppler shifted or otherwise modified version of the transmitted message137 along with spurious signal and/or noise content.

The transmit section 150 may include a signal generator block 158, atransmit beamformer block 156, a summation block 154, and a poweramplifier block 152. The transmit section provides for generation of orfor otherwise obtaining one or more signals or message components 158that will be used to compose a message 137. Notably, a message may becomposed of multiple signals or not. Where a message is composed ofmultiple signals, the message may contain i) signals in parallel(superposed), ii) signals that are serialized (concatenated), or iii)may be a combination of parallel and serial signals.

The transmit beamformer block 156 receives the signal(s) from the signalgenerator block 158 where beamforming for each signal takes place. Thebeam(s) are combined in the summation block 154 to construct a parallel,serial, or combination message M. In the power amplifier block 152, thetime series voltages of the message are amplified in order to excite ordrive the transducers in the projector array 130. In an embodiment, eachtransducer is driven by a respective amplifier.

The receive section 170 includes multiple hydrophone signal processingpipelines. In an embodiment the receive section includes a hardwarepipelines block/analog signal processing block 172, a software pipelinesblock/digital signal processing block 174, a receive beamformer block176, and a processor block 178. The receive section provides forisolating and processing the message 137 from the input 136 received atthe hydrophone array 140. For example, some embodiments process echoesto determine Doppler velocities and/or depths as a function of, amongother things, round trip travel times.

In the hardware pipeline block 172, plural hydrophone array transducersof the hydrophone array 140 provide inputs to plural hardware pipelinesthat perform signal conditioning and analog-to-digital conversion. Insome embodiments, the analog-to-digital conversion is configured foroversampling where the converter Fin (highest input frequency) is lessthan F_(s)/2 (one half of the converter sampling frequency). In anembodiment, a transceiver 122 operates with a maximum frequency of about800 kHz. In an embodiment the transceiver utilizes analog-to-digitalconverters with sampling rates in a range of about 5 to 32 MHz. In anembodiment the transceiver utilizes analog-to-digital converters withsampling rates of about 5 MHz or about 32 MHz.

In the software pipeline block 174, the hardware pipelines 172 provideinputs to the software pipelines. One or more pipelines serve each ofthe hydrophones in the hydrophone array. Each software pipeline mayprovide, among other things, downconversion and/or filtering. In variousembodiments, the software pipeline may provide for recovery of a messagefrom a hydrophone input 136. In an embodiment, hydrophone may be servedby pipelines for one or more of interpreting, distinguishing,deconstructing and/or decoding a message such as a multicomponentmessage.

In the receive beamforming or steering block 176, the software pipelines174 provide beamformer inputs. Beamformer functionality includes phaseshifting and/or time delay and summation for multiple input signals. Inan embodiment, a beamformer is provided for each of multiple codedsignals. For example, where software pipelines operate using two codedsignals, inputs to a first beamformer are software pipelines decoding afirst code and inputs to a second beamformer are software pipelinesdecoding a second code.

In the processor block 178, the beamformers of the beamformer block 176provide processor inputs. Processor functionality may include any one ormore of bottom detection, backscatter processing, data reduction,Doppler processing, acoustic imaging, and generation of a short timeseries of backscatter sometimes referred to as “snippets.”

In an embodiment, a management section 192 and a sensor interfacesection 190 are provided. The management section includes an interfacemodule 194 and/or a workstation computer 196. The sensor interfacesection provides for interfacing signals from one or more sensors ES1,ES2, ES3 such as sensors for time (e.g. GPS), motion, attitude, andsound speed.

In various embodiments, control and/or control related signals areexchanged between the management section 192 and one or more of thepower amplifier block 152, software pipelines block 174, transmitbeamformer block 156, receive beamformer block 176, signal generatorblock 158, processor block 178. And, in various embodiments sensorinterface section data 190 are exchanged with the management section 192and the processor block 178.

FIG. 1D shows portions of an exemplary multibeam echo sounder system(“MBES”) 100D. The echo sounder system includes a transducer section120, a transmitter section 150, and a receiver section 170. Someembodiments include an interface section 190 and/or a management section192.

In the embodiment shown, a message 153 incorporating quantity N signals,for example N coded signals, is used to excite plural projectors in aprojector array and a receiver having quantity T hardware or softwarepipelines and (T*N) hardware or software pipelines may be used toprocess T hydrophone signals for recovery of echo information specificto each of the N coded signals.

In various embodiments, first and second serialized signals within thesame message may be identical as with coded pulse pairs associated withDoppler velocity measurements described below.

The transmitter section 150 is for exciting the projector array 130. Thesection includes a signal generator block 158, a transmit beamformerblock 156, a summation block 154, and a power amplifier block 152.

The signal generator block 158 may generate quantity N signals ormessage components, for example N coded signals (e.g., S_(cd1) . . .S_(cdN)). In some embodiments, each of plural signals within a messageshares a common center frequency and/or a common frequency band. And, insome embodiments, each of plural signals within a message has a unique,non-overlapping frequency band.

A transmit beamformer block 156 receives N signal generator blockoutputs. For each of the N signals generated, the beamformer blockproduces a group of output beam signals such that there N groups ofoutput beam signals.

The summation block 154 receives and sums the signals in the N groups ofoutput beams to provide a summed output 153.

The power amplifier block 152 includes quantity S amplifiers for drivingrespective projectors in the projector array 130. Each power amplifierreceives the summed output or a signal that is a function of the summedoutput 153, amplifies the signal, and drives a respective projector withthe amplified signal.

An array of quantity T hydrophones 140 is for receiving echoes ofacoustic/pressure waves originating from the projector array 130. Theresulting hydrophone signals are processed in the receiver section 170which includes a hardware pipeline block 172, a software pipeline block174, a receive beamformer block 176, and a processor block 178.

In the hardware pipeline block 172, T pipelines provide independentsignal conditioning and analog-to-digital conversion for each of the Thydrophone signals.

In the software pipeline block 174, (T*N) software pipelines may providedownconversion and/or filtering for each of the T hardware pipelineoutputs. Means known in the art, for example filtering such as band passfiltering, may be used to distinguish different signals such as signalsin different frequency bands. As shown, each of T hardware pipelineoutputs 181, 182, 183 provides N software pipeline inputs a,b and c,dand e,f (i.e., 3*2=6 where T=3 and N=2).

In the receive beamformer block 176, (T*N) software pipeline block 174outputs are used to form N groups of beams. A beamformer is provided foreach of N codes. For example, where there are T=3 hydrophones andsoftware pipelines process N=2 codes, inputs to a first beamformer aresoftware pipelines processing the first code a₁, c₁, e₁ and inputs to asecond beamformer are software pipelines processing the second code b₁,d₁, f₁.

In the processor block 178, N processors receive respective groups ofbeams formed by the beamformer block 176. Processor block 178 data areexchanged with a management section 192 and sensor interface 190 dataES1, ES2, ES3 are provided to the management section and/or theprocessor block.

In various embodiments control signals from the management block 192 areused to make power amplifier block 152 settings (e.g., for “S” poweramplifiers for shading), to control transmit 156 and receive 176beamformers, to select software pipeline block 174 operatingfrequencies, and to set signal generator block 158 operatingfrequencies.

As the above illustrates, the disclosed echo sounder transmitter mayconstruct a message incorporating N components such as N coded signals.And, the echo sounder may utilize a receiver having T hardware pipelinesand (T*N) software pipelines to process T hydrophone signals forrecovery of echo information specific to each of the N messagecomponents.

FIGS. 1E-F show portions of an exemplary multibeam echo sounder system(“MBES”) 100E-F. The echo sounder system includes a transducer section120, a transmitter section 150, and a receiver section 170. Someembodiments include an interface section 190 and/or a management section192.

In the embodiment shown, a message 153 incorporating first, second, andthird message components such as coded signals S_(cd1), S_(cd2), S_(cd3)where N=3 is used to excite three projectors in a projector array, and areceiver having three hardware pipelines and nine software pipelines isused to process three hydrophone signals T=3 to recover echo informationspecific to each of the N message components.

The transmitter section 150 is for exciting the projector array 130. Thesection includes a signal generator block 158, a transmit beamformerblock 156, a summation block 154, and a power amplifier block 152.

In the signal generator block 158, signals are constructed, generated,recalled and/or otherwise provided. Here, an exemplary process isdepicted with e.g., N=3 signal generators. In respective beamformers ofthe beamformer block 156, multiple beams are generated from each signal.In a summation block 154, the beams are combined to produce a summationblock output signal or transmit message 153.

The transducer block 120 includes a projector array 130 and a hydrophonearray 140 arranged, for example, as a Mills Cross. As shown, there arethree projectors 131 in the projector array and three hydrophones 141 inthe hydrophone array. In the power amplifier block 152, the summedsignal or transmit message 153 is an input to power amplifiers drivingrespective projectors.

Applicant notes that for convenience of illustration, the projector andhydrophone counts are limited to three. As skilled artisans willappreciate, transducer arrays do not require equal numbers of projectorsand hydrophones nor do the quantities of either of these types oftransducers need to be limited to three. For example, a modern multibeamecho sounder might utilize 1 to 96 or more projectors and 64 to 256 ormore hydrophones.

The array of T=3 hydrophones 141 is for receiving echoes resulting fromthe acoustic/pressure waves originating from the projector array 130.The resulting hydrophone signals are processed in the receiver section170 which includes a hardware pipeline block 172, a software pipelineblock 174, a receive beamformer block 176, and a processor block 178.

In the hardware pipelines block 172, each of T=3 hardware pipelinesprocesses a respective hydrophone 141 signal through analog componentsincluding an analog-to-digital converter. In the embodiment shown, ahardware pipeline provides sequential signal processing through a firstamplifier, an anti-aliasing filter such as a low pass anti-aliasingfilter, a second amplifier, and an analog-to-digital converter.

In the software pipelines block 174, each of the T=3 hardware pipelineoutputs is processed through N=3 software pipelines with downconversionand band pass filtering. In the embodiment shown, a software pipelineprovides sequential signal processing including processing through amixer (an oscillator such as local oscillator may be coupled to themixer), a bandpass filter, and a decimator. Communications may occur viacommunications links between any of the processor block 178, the signalgenerator block 158, the hardware pipelines block 172, the softwarepipelines block 174, the and the beamformer block 176. See for exampleFIGS. 1C-D.

Each software pipeline may have a single mixer and/or each hardwarepipeline may have no mixer. A processor 178 may control gain of a firstand/or a second hardware pipeline amplifier. A processor may provide fortuning, for example via a processor controlled oscillator coupled with amixer.

In the receive beamformer block 176, each of N=3 beamformers processessignals. As such, i) a first set of three software pipeline outputscorresponding to a first coded signal are processed by a firstbeamformer, ii) a second set of three software pipeline outputscorresponding to a second coded signal are processed by a secondbeamformer, and (iii) a third set of three software pipeline outputscorresponding to a third coded signal are processed by a thirdbeamformer. Notably, beamformers may be implemented in hardware orsoftware. For example, one or more beamformers may be implemented in oneor more field programmable gate arrays (“FPGA”).

In the processor block 178, each of N=3 processors are for processingrespective beamformer outputs. Here, a first plurality of beamsgenerated by the first beamformer is processed in a first processor, asecond plurality of beams generated by the second beamformer isprocessed in a second beamformer, and a third plurality of beamsgenerated by the third beamformer is processed in a third beamformer.

Processor outputs interconnect with a management section 192. Notably,one or more processors may be implemented in a single device such as asingle processor or digital signal processor (“DSP”) or in multipledevices such as multiple signal processors or digital signal processors.

Complementary data may be provided via, inter alia, a sensor interfacesection 190 that is interfaced with a plurality of sensors ES1, ES2,ES3. The sensor interface module may provide sensor data to themanagement section 192 and/or to processors in the processor block 178.

The management section 192 includes a sonar interface 194 and/or aworkstation computer 196. In various embodiments control signals fromthe management block 192 are used for one or more of making poweramplifier block 152 settings (e.g., for array shading), controllingtransmit beamformers 156 and receive beamformers 176, selecting softwarepipeline block 174 operating frequencies, setting set signal generatorblock 158 operating frequencies, and providing processor block 178operating instructions.

Applicant notes that the echo sounder systems of FIGS. 1C-F may be usedto process hydrophone returns from targets i) present within anensonified volume of the water body, ii) upon an ensonified surface ofthe bottom, or iii) lying within an ensonified volume of the bottom.

In various embodiments, the MBES of FIGS. 1E-F distinguishes amongsignals based on frequency or frequency band. In various embodiments,the MBES of FIGS. 1E-F does not distinguish among signals using matchedfiltering.

MBES Message Cycle

FIG. 2A shows a first message cycle 200A. The cycle includes a sequenceof operations with transmission of a message during a time t1 andreception of a message during a time t3. Transmission of a messagerefers to a process that excites the projector array 130 and receptionof a message refers to a complementary process including message echoreceipt by the hydrophone array 140. A wait time t2 that variesprimarily with range, angle, and sound speed may be interposed betweenthe end of the message transmission and the beginning of the messagereception. This wait time may be determined by the sonar range scalesetting or round trip travel time for the longest sounding range, forexample a return from the most distant observed location or cell in aswath ensonified by the projector array. In some embodiments, themessage transmit length is in a range of 10 to 60 microseconds. In someembodiments, the transmit message length is about 5-15 milliseconds or10 milliseconds.

FIG. 2B shows a second message cycle 200B. Here, a transmitted messageincludes multiple coded message components. During transmission of themessage, each of the message components is steered as by beamformers 156to ensonify a respective strip or fan of a waterbody bottom as isfurther explained below. Each of the transmitted message componentsresults in a similarly coded message component return. Decoding in thereceiver separates these returns such that data specific to each fan isavailable for analyses.

Multimode Doppler Operations

As mentioned above, the MBES disclosed in FIGS. 1A-F may be operated inmultiple modes. These modes include various Doppler Velocity Log (“DVL”)modes and various Acoustic Doppler Current Profiling (“ADCP”) modes.

When operating the MBES in a typical DVL navigation mode, the MBES maybe placed on a moving platform such as a surface vessel for targeting astationary reflector such as a waterbody bottom.

When operating the MBES in a typical ADCP current profiling mode, theMBES may be placed on a stationary platform such as a waterbody bottomfor targeting reflectors and/or scatterers entrained in a moving watercolumn.

As skilled artisans will appreciate, acoustic Doppler measurements maybe used to determine velocity and the velocity determined may be,whether in a DVL or an ADCP mode, a relative velocity between the MBESand the reflector or source of the backscattered acoustic energy.

Multifan Operations

FIGS. 3A-D show an exemplary vessel equipped with a multi-beam echosounder 300A-D. See for example the echo sounders of FIGS. 1A-E. As seenin FIG. 3A, an MBES array package 304 is affixed to a vessel 302, forexample to a bottom of the vessel.

Within the array package 304 is an along track array of projectors 308and a cross track array of hydrophones 310. The projector array is forexcitation by a transmit message such as the message of FIG. 2A or FIG.2B. The hydrophone array is for receiving echoes of the transmittedmessage. As explained below, a crossed array arrangement such as a MillsCross arrangement of the projector and hydrophone arrays enables theecho sounder to operate with crossed transmit and receive beams whereinthe cross intersection identifies a particular waterbody location, area,or cell. The crossed arrays may be in a perpendicular or a substantiallyperpendicular arrangement. Substantially perpendicular refers togenerally small deviations from perpendicular caused by any of arrayassembly tolerances, mounting tolerances, adjustment tolerances, and thelike.

FIG. 3B shows bottom ensonification 300B. In particular, an across trackstrip or fan of a waterbody bottom 312 is ensonified by the projectorarray 308. Note the along track projector array 308 ensonifies an acrosstrack fan. As shown, the projected beam 311 has a wide across trackaperture angle θ_(t1) as compared with a relatively narrow along trackaperture angle θ_(t2). Echoes from this ensonified fan may be receivedby the hydrophone array 310.

DVL bottom-tracking mode: In light of the multimode Doppler operationsdiscussed above, it will be appreciated that FIGS. 3A-B illustrateoperation of an MBES in a DVL bottom-tracking mode where the MBES ismoving and the reflector(s), for example reflectors on a sea floor, areassumed to be stationary. DVL water-tracking mode: Another configurationcalled DVL water-tracking mode exploits backscatter from a moving waterlayer within the water column to estimate a moving MBES' relativevelocity through the water. ADCP mode: Additionally, if the MBES ofFIGS. 3A-B is stationary and oriented such that a vertical strip of thewater column is ensonified from above or below, the MBES mode ofoperation becomes an ADCP mode. Each of FIGS. 3C-F may be viewed in asimilar manner to visualize multifan ensonification of the water columnduring an ADCP or DVL water-tracking mode of operation.

FIG. 3C shows bottom ensonification and echoes that result from thebottom 300C. In particular, echoes from the ensonified across track fan312 are received by the hydrophone array 310. As shown, the receivedbeam 321 has a wide along track aperture angle θ_(r1) as compared with arelatively narrow across track angle θ_(r2). And, as shown, thehydrophone array beam may be steered to observe or read a set of alongtrack strips 331, 332, 333 . . . that intersect the ensonified fan 312at multiple adjacent or overlapping locations. Data such as bathymetricdata may be obtained from and associated with each of these intersectinglocations or areas 340 such that each time an across track fan isensonified, multiple receiving beams observe multiple receiving stripsand provide bathymetric data at multiple locations along the ensonifiedfan.

Just as a single ensonified fan 312 may be observed or read by multiplereceiving beams 321, so too may multiple ensonified fans be observed orread by multiple receiving beams.

FIG. 3D shows multifan bottom ensonification 300D. Here, the projectorarray is steered to produce multiple adjacent or overlapping ensonifiedstrips or fans that are oriented across track. While any number of fans,such as 2, 3, 4, 5, 10 or more fans, may be projected, the example ofFIG. 3D shows five projected fans comprising a center fan flanked byForward A and Aft A fans which are flanked by Forward B and Aft B fansrespectively. As before, multiple receiving beams 351 provide a set ofalong track receiving strips 361, 362, 363. These receiving stripsintersect the multiple fans 372.

When a receiving strip 362 intersects multiple fans, a plurality 372 ofcells 340 may be observed. And, when multiple receiving strips 361, 362,363 . . . intersect multiple fans, a grid-like or two dimensional zone370 results and bathymetric data may be obtained from each of the cellsidentified by intersections within the zone.

Applicant notes that as shown in FIG. 3D each of the fans has opposedcross-track boundaries that are essentially straight lines. Thispresentation is idealized. In practice, these opposed fan boundaries maybe curved. For example, fan outlines on a waterbody bottom may beparabolic in shape with a cross-track major dimension. Transmitbeamforming and/or other than planar waterbody bottoms may contribute tofans having other than straight cross-track boundaries but that does notpreclude locating the centers of the cells 340.

Advantages of multifan operation may include increased survey speedresulting from, for example, an extended along track zone ofensonification, redundancy via overlapping of zones (e.g., where a fiftypercent overlap between pings may provide two looks at every waterbodybottom location observed), and imaging a given target from multipleaspects. For example, imaging from multiple aspects including at nadirand from two opposing off-nadir sides. For example, imaging frommultiple aspects including front, overhead, and behind.

In various embodiments, realizing the benefits of a multifan surveysystem requires an MBES capable of distinguishing between echoesreturned from each of the fans. And, in various embodiments, any oftemporal, spectral, or code separation techniques may be used to relatean echo to the fan from which it originated. In some embodiments,frequency separation is used to associate returns with particular fansas is further explained below. And in an embodiment, temporal separationis used to distinguish message components in a multicomponent message.And, in an embodiment, temporal separation is used to distinguishmessage components in a multicomponent message transmit over one or moremessage cycles.

FIG. 3E shows a transmitted message ensonifying five fans 300E. Here, anMBES projector array 308 transmits 380 five formed beams 381-385 to acenter fan, to Aft A and Forward A fans flanking the center fan, and toperipheral Aft B and Forward B fans. Each of the five formed beams381-385 ensonifies a respective fan with one of five differing frequencyband signals.

In the example shown, Aft B fan is ensonified with signal 1 in frequencyband A by the first beam 381, the Aft A fan is ensonified with signal 2in frequency band B by the second beam 382, the Center fan is ensonifiedwith signal 3 in frequency band C by the third beam 383, the Forward Afan is ensonified with signal 4 in frequency band D by the fourth beam384, and the Forward B fan is ensonified with signal 5 in frequency bandE by the fifth beam 385. Notably, five messages may be sent in fivedifferent frequency bands to ensonify the five fans. The messages may besent concurrently and separated in the receiver by frequency band.

FIG. 3F shows returns 300F from the ensonified fans of FIG. 3E. Here, anMBES hydrophone array 310 receives returns 390 from the center fan 393,from Aft A and Forward A fans flanking the center fan 392, 394, and fromperipheral Aft B and Forward B fans 391, 395.

It is noted that in some embodiments, one or multiple projector and/orhydrophone arrays may be used in connection with multifan operations.For example, multiple projectors or projector arrays might be used tosimultaneously ensonify fans in multiple fixed look directions. Forexample, multiple hydrophones or hydrophone arrays might be used tosimultaneously acquire returns from multiple fixed look directions.

Acoustic Doppler Measurements

As shown above, an MBES may be designed, built, and operated to ensonifymultiple fans. For example, a 256 beam system that ensonifies three fanscan acquire data from 3*256 beams. In the case of bathymetry and datafrom 3*256 waterbody bottom locations, this multiplicity of measurementsmay be used, for example, to improve survey speed and/or the density ofsurvey measurements. In the case of navigation, this multiplicity ofmeasurements may be used, for example, to improve the accuracy ofvelocities determined using acoustic Doppler techniques.

Where a source emits acoustic signals and a target moving relative tothe source reflects the signals, acoustic Doppler techniques may be usedto determine a velocity of the target relative to the source. Forexample, changes in acoustic wavelength of reflected signals (e.g.,returns, echoes) may be used to determine a radial component ofvelocity. In some cases, the source is fixed and the target is moving.And, in some cases, the target is fixed and the source is moving.

FIG. 4A shows a flowchart of steps that may be used in determiningvelocities from multifan data 400A. In a first step 402, a message isconstructed. The message is for transmission from an MBES source movingrelative to a waterbody bottom at velocity V_(source). In variousembodiments, the message includes a pulse pair such as a pair ofidentical pulses.

In a subsequent step 404, the message is used to ensonify each of “i”fans with “j” beams. Thereafter, in step 406, returns are processed andvelocities, for example Doppler radial velocities DRV_(i,j) associatedwith the beams, are determined. And, in a subsequent step 408, theDoppler radial velocities DRV_(i,j) along with known variables such asfan and beam geometry are used to determine a velocity vector that isrepresentative of V_(source).

Notably, where each Doppler radial velocity DRV_(i,j) is a projection ofV_(source) along beam j, each of the Doppler radial velocities can besaid to be indicative of V_(source).

In light of the multimode Doppler operations discussed above, it will beappreciated that FIG. 4A illustrates operation of an MBES in a DVLbottom-tracking mode where the MBES is moving and the reflector(s) arestationary. However, if the backscatter originates from a layer of thewater column, the DVL operates in water-tracking mode. Additionally, ifthe MBES of FIG. 4A is made stationary and directed to target scatteringcenters in the water column, the MBES operates in an ADCP mode.

FIGS. 4B-C show a moving source 400B-C. As seen in FIG. 4B, a boat 403carries a hull mounted MBES 405. The MBES 405 has a heading 416. Aheading coordinate system with orthogonal axes x′, y′, z′ is shown inFIG. 4C. The heading coordinate system has an origin centered on thesource 414 and an x′ axis aligned with the heading 416. Message beamsemanating from the source are directed with fan angles in the x′z′ planeFA_(i) and beam projection angles in the x′y′ plane BA_(j). Multiplebeams within a fan do not need to be equally spaced within the sectorthey span, nor do fans need to be evenly distributed fore and aft. Thereis no requirement to use a fan at nadir, although in an embodiment wherethree fans cover a 40 degree arc in the x′z′ plane, fan angles measuredfrom the z′ axis in the x′z′ plane are −20, 0 and +20 degrees.

FIGS. 4D-F show a course coordinate system 400D-F. This coursecoordinate system is centered on the source 414 and is described byorthogonal axes x, y, z. As seen in FIG. 4D, motion of the source mayinclude one or more of rolling about the x axis, pitching about the yaxis, and yawing about the z axis.

In FIG. 4E, a yaw angle YA describes the angular offset measured in thexy plane. The yaw angle, also commonly referred to as crab angle, is theangle between the course direction along the x axis and the headingdirection along the x′ axis. Here, the x axis lies along the line thatis the course or instantaneous course of the boat 412. As skilledartisans will appreciate, in the absence of external forces on the boat,the course and heading may not differ. However, when an external force,e.g., wind, waves, or current, acts on the boat, the heading will beoffset from the course to compensate for the forces which tend to movethe boat off course.

In FIG. 4F, a pitch angle PA describes an angular offset measured in thexz plane. The pitch angle is measured from the z axis and indicates thepitch of the boat. Notably, the location of a fan on a waterbody bottomvaries with fan angle FA setting and with boat pitch angle PA.

As mentioned above, a multiplicity of Doppler velocities such as amultiplicity of Doppler radial velocities DRV_(i,j) may be used toestimate and/or determine a relative velocity between an MBES sourceV_(source) and a reflector. Notably, the reflector may be a waterbodybottom, scattering centers entrained in the water column such as bubblesor particulate, and reflectors otherwise located in the water column.Notably, where the MBES is mounted on a vessel for DVL bottom tracking,V_(source) is a source velocity relative to a stationary waterbodybottom target. Where the MBES is mounted on a vessel for DVL watertracking, V_(source) is a source velocity relative to a water layer thatmay be moving in the water column. And, where the MBES is mounted on astationary vessel or waterbody bottom for ADCP water columnmeasurements, V_(source) is water column target velocity relative to astationary MBES.

FIGS. 4G-I show exemplary equations 400G-I to solve for V_(source) usinga plurality of Doppler radial velocities, each Doppler radial velocitybeing indicative of V_(source). Notably, applicants have found that suchestimates of V_(source) may be more accurate than estimates based on asingle or only a few beams and/or may be more accurate than estimatesbased on returns from a single or only a few waterbody bottom or watercolumn locations.

Determination of DRV_(i,j) may utilize the pulse-pair method, anefficient computational algorithm known to skilled artisans, to processdata from each fan i and beam j individually (See e.g. U.S. Pat. No.5,483,499). A complex representation (angle and magnitude) of theautocorrelation of beam data at a time lag equal to one pulse length Tis calculated for all range cells k, one of which is selected, dependingon the operating mode of the MBES, to provide angle informationCSANGLE_(i,j) to the calculation of DRV_(i,j) as follows:DRV_(i,j)=CSANGLE_(i,j)*(c/(2*pi*f _(c) *T))where c=speed of sound (m/s)

-   -   f_(c)=center frequency of transmitted pulse (Hz)    -   T=pulse length (s)

As seen, Doppler radial velocity DRV_(i,j) varies with fan i and beam j,and there are a total of (i*j) DRV estimates. In various embodiments,all of these DRV estimates contribute to an estimate of V_(source). And,in various embodiments only selected ones of these DRV estimatescontribute to an estimate of V_(source). For example, in a beam skippingembodiment, pairs of selected beams in a particular fan may be separatedby an integer quantity of p beams that are not selected such that wherep=2, beams 1, 4, 7 . . . are selected and beams 2, 3, 5, 6 are notselected. For example, in a first reduced beam count embodiment a sourcevelocity is estimated using a quantity r1<(i*j) of the DRV_(i,j)estimates and r1 is determined in part by the processing capacity of adigital processing section of the receiver. For example, in a secondreduced beam count embodiment, the value of r1 is automaticallydetermined by the acoustic Doppler system as a function of equipmentvariables, environmental variables, and/or mission requirements.

The quantity V_(source) is estimated via minimization of a cost equation400H such as the cost equation of FIG. 4H. FIG. 4I shows cost equationvariables 400I that are knowns DRV_(i,j), BA_(j), FA_(i) and costequation variables that are unknown V_(source), YA, and PA. As skilledartisans will appreciate, the hypothesized and/or estimated values forthe unknowns V_(source), YA, and PA that minimize cost and/or result inthe smallest cost are declared the best estimates for those values,together describing a source velocity vector in polar coordinates. Costequation minimization methods include brute force searches, Nelder-Meadtype methods, Newton's method, and other suitable minimization methodsknown to skilled artisans.

Summed for a plurality of beams j in each of a plurality of fans i, thecost equation is

${Cost} = {\sum\limits_{i = 1}^{fans}{\sum\limits_{j = 1}^{beams}\left( {{{kx}\; 1*V_{source}*D} - {DRV}_{i,j}} \right)^{2}}}$where: D=Cos (YA−BA_(j))*Cos (PA−FA_(i))

-   -   k×1=2 for DVL mode    -   k×1=1 for ADCP mode    -   V_(source)=Relative velocity between MBES and        reflector/scatterer

Notably, constant k×1 recognizes that for typical DVL modes the sourceis in motion and therefore its echo is Doppler shifted twice, once ontransmit and once on receive. Hence the value k×1=2 for DVL mode andk×1=1 for ADCP mode.

Exemplary DVL Process

FIG. 4J shows a flowchart 400J of an exemplary DVL process using pulsepairs to determine source velocity. Here, a source for emitting anacoustic message with consecutive components is provided 482. Thesource, such as a vessel mounted source 405, may be subjected to pitchPA and yaw YA while moving along a course 484. As skilled artisans willappreciate, a source heading may differ from the course by a yaw angleYA 486.

As the source moves along the course, it ensonifies a waterbody bottomwith multiple fans F_(i) at fan angles FA_(i) with multiple beams B_(ij)at beam angles BA_(ij) 488. In a step 490 that follows, returns from theensonified fans are received, time gated, and associated with rangecells along each beam in each fan RC_(i,j,k). See for example themultibeam echo sounder systems of FIGS. 1B-F.

In a step 492 that follows, application of the pulse-pair methodprovides a complex autocorrelation value with an angle CSANGLE_(i,j,k)and magnitude CSMAG_(i,j,k) for each range cell RC_(i,j,k) (see. e.g.,U.S. Pat. No. 5,483,499).

In a step 494 that follows, the range cell in each beam that correspondsto the waterbody bottom is determined. For example, the maximumcorrelation magnitude CSMAG_(i,j,k) in each beam may be used to identifya waterbody bottom range cell WBBRC_(i,j) in each beam.

In a step 496 that follows, Doppler radial velocities are calculated foreach range cell corresponding to the waterbody bottom. For example,CSANGLE_(i,j,k) may be used to calculate Doppler radial velocityDRV_(i,j) for each WBBRC_(i,j).

In a step 498 that follows, source velocity is estimated. For example,the cost equation mentioned above, with k×1=2 for DVL mode, may beminimized to estimate unknowns V_(source), YA, and PA given knownsDRV_(i,j) BA_(j), and FA_(i).

Exemplary ADCP Process

FIG. 4K shows a flowchart 400K of an exemplary ADCP process using pulsepairs to determine source velocity. Here, a source for emitting anacoustic message with consecutive components is provided 482. The sourceis considered stationary 485, such as one mounted on the sea floor or toa non-moving vessel. Water layers above or below the source,respectively, may be in motion relative to the source, and each layermay have a velocity independent of the other layers 487. The stationarysource ensonifies a water column with multiple fans F_(i) at fan anglesFA_(i) with multiple beams B_(ij), at beam angles BA_(ij) 488. In a step490 that follows, returns from the ensonified fans are received, timegated, and associated with range cells along each beam in each fanRC_(i,j,k). See for example the multibeam echo sounder systems of FIGS.1B-F.

In a step 492 that follows, application of the pulse pair methodprovides a complex autocorrelation value with an angle CSANGLE_(i,j,k)and magnitude CSMAG_(i,j,k) for each range cell RC_(i,j,k). In a step495 that follows, for each beam in each fan a water column range cell ata depth of interest WCRC_(i,j) is selected. These range cells likelycapture part of the water column or a water volume instead of thewaterbody bottom. Further, although only one depth of interest may beindicated, the process can be repeated for numerous depths tocollectively form a vertical profile of velocity estimates and/or anaverage velocity estimate.

In a step 497 that follows, Doppler radial velocities DRV_(i,j) arecalculated for each range cell corresponding to the depth of interest.For example, CSANGLE_(i,j,k) may be used to calculate DRV_(i,j) for eachWCRC_(i,j).

In a step 499 that follows, source velocity is estimated. For example,the cost equation mentioned above, with k×1=1 for ADCP mode, may beminimized to estimate unknowns V_(source), YA, and PA given knownsDRV_(i,j), BA_(j), and FA_(i).

FIGS. 5A-F below show exemplary message constructs 500A-F. One or moreof these message constructs may be propagated through a liquid medium togenerate returns from one or more fans to estimate a velocity of thesonar system sending the message and/or the velocity of a vessel thatcarries the sonar system.

FIG. 5A shows a single fan message including a pulse pair 500A. A firstmessage component includes message code 1 transmitted during a timespantx1 and a second message component includes the same message code 1transmitted during a timespan tx3. These two timespans and similartimespans discussed below may be separated by a timespan tx2 or they maybe contiguous such that tx2=0. The timespan between the beginning of thefirst message component and the end of the second message component istx4. In various embodiments, pulses in one or more pulse pairs do notoverlap in time.

As shown, the message components may be sent using a particulartransmitter frequency band. This transmitter frequency band may be lessthan (as shown) or substantially equal to an available frequency band ofa multibeam echo sounder transmitter 150 and/or a multibeam echo sounderreceiver 170.

FIG. 5B shows a multi-fan message including three pulse pairs forensonifying three fans simultaneously or substantially simultaneously500B. Here, the message includes pulse pairs transmitted in parallel andserially transmitted pulses within the each pulse pair. The message maytherefore be referred to as a serial-parallel message or aserial-parallel message using single pulse pairs insofar as the pulsesof the pulse pairs are transmitted serially and the fans are ensonifiedsimultaneously.

For a first fan (Fan 1), message components include message code 1 andmessage code 1 transmitted serially in a first receiver frequency bandA. For a second fan (Fan 2), message components include message code 2and message code 2 transmitted serially in a second receiver frequencyband B. For a third fan (Fan 3), message components include message code3 and message code 3 transmitted serially in a third receiver frequencyband C.

In some embodiments, message codes 1, 2, 3 of the first messagecomponent for each of the fans are transmitted simultaneously in each ofthree non-overlapping and/or contiguous receiver frequency bands. And,in some embodiments message codes 1, 2, 3 of the second messagecomponent for each of the fans are transmitted simultaneously in each ofthe three non-overlapping and/or contiguous receiver frequency bands.

FIG. 5C shows a multi-fan message including three pulse pairs forensonifying three fans in sequence 500C. Here, the message includesserially transmitted pulse pairs and serially transmitted pulses withinthe each pulse pair. The message may therefore be referred to as aserial-serial message using single pulse pairs insofar as the pulses ofpulse pairs are transmitted serially and the fans are ensonifiedserially.

Skilled artisans will recognize serial messages may increase thestrength of the signal ensonifying each fan because transmitter signalstrength is not shared among fans as may happen with messages havingcomponents transmitted in parallel.

For a first fan, message components include message code 1 and messagecode 1 transmitted serially in a first receiver frequency band A. For asecond fan, message components include message code 2 and message code 2transmitted serially in a second receiver frequency band B. For a thirdfan, message components include message code 3 and message code 3transmitted serially in a third receiver frequency band C.

As seen, each of the fans is ensonified in sequence as a fan 1 pulsepair transmitted in band A is followed by a fan 2 pulse pair transmittedin band B which is followed by a fan 3 pulse pair transmitted in band C.In various embodiments, the fan ensonifying messages do not overlap andin various embodiments the fan ensonifying messages are contiguous. Thetransmission frequency bands may be non-overlapping and/or contiguousreceiver frequency bands. The messages may be transmit in one or moremessage cycles.

Disambiguation

Where a single pulse pair is used in each frequency band as shown inFIG. 5B, source velocity accuracy and/or ambiguity issues may arise. Forexample, it is known that longer pulses can provide greater accuracywhen using e.g. pulse pair correlation methods. However, longer pulsesmay result in ambiguous source velocity estimates because longer pulsesmay experience phase shifts exceeding a full 2 pi rotation. This phasewrapping ambiguity may result in erroneous source velocity estimates.

As seen below, both short and long pulse pairs in each frequency bandmay be used to resolve ambiguous long pulse measurements. Here, a shortpulse pair provides an initial estimate within 2 pi of the long pairphase shift, and this initial estimate is used to resolve any ambiguityin a corresponding long pulse-pair estimate.

In FIG. 5D, a multifan message utilizes dual pulse pairs transmitted inparallel 500D. In particular, for each of n=3 fans, a dual pulse messageis used to ensonify the fan such that 2n=6 pulses are transmitted in themessage. Here, the 3 dual pulse pairs are transmitted simultaneouslywhile the message components within each dual pulse pair are transmittedserially. The message may therefore be referred to as a serial-parallelmessage or a dual pulse pair serial-parallel message insofar as thepulses of the dual pulse pairs are transmitted serially and the fans areensonified simultaneously.

For a first fan, message components for a short pulse pair includemessage code 11 and message code 11 while message components for a longpulse pair include message code 12 and message code 12. As such, thefirst fan may be ensonified by a short pulse pair and a long pulse pair.These pulse pairs may be transmitted in band A.

For a second fan, message components for a short pulse pair includemessage code 21 and message code 21 while message components for a longpulse pair include message code 22 and message code 22. As such, thesecond fan may be ensonified by a short pulse pair and a long pulsepair. These pulse pairs may be transmitted in band B which may or maynot be contiguous with band A.

For a third fan, message components for a short pulse pair includemessage code 31 and message code 31 while message components for a longpulse pair include message code 32 and message code 32. As such, thethird fan may be ensonified by a short pulse pair and a long pulse pair.These pulse pairs may be transmitted in band C which may or may not becontiguous with band B.

As seen, there are n fans and 2n pulse pairs including i) n short pulsepairs, each pair including first and second message components and ii) nlong pulse pairs, each pair including third and fourth messagecomponents.

In some embodiments, short pulse message codes 11, 21, 31 of the firstmessage component for each of the fans are transmitted simultaneously ineach of three non-overlapping and/or contiguous receiver frequencybands. And, in some embodiments, short pulse message codes 11, 21, 31 ofthe second message component for each of the fans are transmittedsimultaneously in each of the three non-overlapping and/or contiguousreceiver frequency bands. In some embodiments, long pulse message codes12, 22, 32 of the third message component for each of the fans aretransmitted simultaneously in each of three non-overlapping and/orcontiguous receiver frequency bands. And in some embodiments, long pulsemessage codes 12, 22, 32 of the fourth message component for each of thefans are transmitted simultaneously in each of the three non-overlappingand/or contiguous receiver frequency bands.

In FIG. 5D and in FIG. 5E below, embodiments include those where i)message codes 11, 21, 31 are g bit codes and each bit is u samples longand ii) message codes 12, 22, 32 are g bit codes and each bit is aninteger multiple of u samples long, for example 4u samples long. Forexample, when u=1 and where 13-bit Barker codes are used with no timedelay between codes, 130 samples (13+13+52+52) at a sample rate of68,400 Hz results in a message that is 1.9 milliseconds long.

Applicant notes this and other examples may suggest use of a limitednumber of fans, for example three fans. However, no such limitation isintended. Rather, survey system hardware 100B may support larger fanarrays such as arrays of 5, 7, 10, 20, 40, or more fans.

While various codes known to skilled artisans might be used inconstructing message components, the inventor's experience suggest thatBarker codes and Orthogonal Spread Spectrum (“OSS”) codes are suitablealternatives in many applications of interest.

In FIG. 5E, a multifan message utilizes dual pulse pairs 500Etransmitted in sequence. In particular, for each of n=3 fans, a dualpulse message is used to ensonify the fan such that 2n=6 pulses aretransmitted in the message. Here, the 3 dual pulse pairs are transmittedsequentially and the message components within each dual pulse pair aretransmitted serially. The message may therefore be referred to as aserial-serial message or a dual pulse pair serial message insofar as thepulses of dual pulse pairs are transmitted serially and the fans areensonified serially.

For a first fan, message components for a short pulse pair includemessage code 11 and message code 11 while message components for a longpulse pair include message code 12 and message code 12. As such, thefirst fan may be ensonified by a short pulse pair and a long pulse pair.These pulse pairs may be transmitted in band A.

For a second fan, message components for a short pulse pair includemessage code 21 and message code 21 while message components for a longpulse pair include message code 22 and message code 22. As such, thesecond fan may be ensonified by a short pulse pair and a long pulsepair. These pulse pairs may be transmitted in band B which may or maynot be contiguous with band A.

For a third fan, message components for a short pulse pair includemessage code 31 and message code 31 while message components for a longpulse pair include message code 32 and message code 32. As such, thethird fan may be ensonified by a short pulse pair and a long pulse pair.These pulse pairs may be transmitted in band C which may or may not becontiguous with band B.

As seen, there are n fans and 2n pulse pairs including n short pulsepairs and n long pulse pairs.

As seen, each of the fans is ensonified in sequence as a fan 1 dualpulse pair transmitted in band A is followed by a fan 2 dual pulse pairtransmitted in band B which is followed by a fan 3 dual pulse pairtransmitted in band C. In various embodiments, the fan ensonifyingmessages do not overlap and in various embodiments the fan ensonifyingmessages are contiguous. The transmission frequency bands may benon-overlapping and/or contiguous receiver frequency bands. The messagemay be transmit in one or more message cycles.

In some embodiments, Barker codes with lengths of one or more of 2, 3,4, 5, 7, 11, and 13 bits are used to construct single pulse pair X-Xmessages and/or dual pulse pair messages X-X, Y-Y.

For example, where each of qty. xb bits in message code X are expressedwith xs samples and each of qty. yb bits in message code Y are expressedwith ys samples, an exemplary short message code X might be an xb=11 bitBarker code with xs=1 sample per bit while a long message code Y mightbe the same code with yb=11 and a greater number of samples ys=4 samplesper bit. Note that in a dual pulse pair message X-X, Y-Y codes, the codein message code X may differ from the code in message code Y and thenumber of samples used to express each bit in message code X may differfrom the number of samples used to express each bit in message code Y.

Yet another message may include short message code X using, for example,a 7 bit Barker code and a long message code Y using, for example, a 13bit Barker code. In various embodiments, the number of samples used toexpress bits in message code X may be the same or different from thenumber of samples used to express bits in the message code Y so long as(xs*xb)<(ys*yb). In some embodiments, (xs*xb)>(ys*yb).

In some embodiments, the pulses of pulse pairs may be ordered such thatshort pulse pairs are transmitted first or such that long pulse pairsare transmitted first. In some embodiments pulses used to construct apulse pair may be transmitted as concatenated pulses or transmitted witha time delay therebetween to the extent that the velocity estimatesremain substantially concurrent in time.

FIG. 5F shows various code selections for use in a three fan Dopplervelocity measurement 500F. For each of the fans, the message includes ashort code pulse pair and a long code pulse pair.

Here, fan 1 message components include a Barker Code short pulse pair(Barker Code 11, Barker Code 11) and a Barker Code long pulse pair(Barker Code 12, Barker Code 12) in frequency band A. Fan 2 messagecomponents include a Barker Code short pulse pair (Barker Code 21,Barker Code 21) and a Barker Code long pulse pair (Barker Code 22,Barker Code 22) in frequency band B. Fan 3 message components include aBarker Code short pulse pair (Barker Code 31, Barker Code 31) and aBarker Code long pulse pair (Barker Code 32, Barker Code 32) infrequency band C.

Frequency bands A, B, C may be non-overlapping and/or contiguous. Insome embodiments, for each fan, the short and long pulse pairs arecontiguous. In some embodiments, these pulse pairs are not contiguous.

As skilled artisans will appreciate, when a particular fan (e.g., 1, 2,3) and pulse-pair type (e.g., 1:short, 2:long) uses a particular Barkercode (e.g., Barker Code (fan, type)=Barker Code 11 for fan 1, type 1),signals returned from a particular fan may be distinguished by frequencywhile in-band signals (e.g., short pulse pair [Barker Code 1, BarkerCode 1] and long pulse pair [Barker Code 4, Barker Code 4]) may betemporally non-overlapping and/or temporally separated.

In an embodiment, Orthogonal Spread Spectrum (“OSS”) codes are used.Here, an exemplary short message code might be an OSS code n sampleslong while a long message code might be an OSS code that is greater thann samples long.

Here fan 1 message components include an OSS short pulse pair (OSS Code11, OSS Code 11) and an OSS long pulse pair (OSS Code 12, OSS Code 12)in frequency band A. Fan 2 message components include an OSS short pulsepair (OSS Code 21, OSS Code 21) and an OSS long pulse pair OSS Code 22,OSS Code 22) in frequency band B. Fan 3 message components include anOSS short pulse pair (OSS Code 31, OSS Code 31) and an OSS long pulsepair (OSS Code 32, OSS Code 32) in frequency band C.

In some embodiments, for each fan, the short and long pulse pairs arecontiguous. In some embodiments, these pulse pairs are not contiguous.

As skilled artisans will appreciate, when a particular fan (e.g., 1, 2,3) and pulse pair type (e.g., 1:short, 2:long) uses a particular OSScode (e.g., OSS Code (fan, type)=OSS Code 11 for fan 1, type 1), signalsreturned from a particular fan may be distinguished by frequency. Invarious embodiments in-band signals (e.g., short pulse pair [Code X,Code X] and long pulse pair [Code Y, Code Y]) may be temporallynon-overlapping and/or temporally separated.

While various embodiments of the present invention have been describedabove, it should be understood that they have been presented by way ofexample only, and not limitation. It will be apparent to those skilledin the art that various changes in the form and details can be madewithout departing from the spirit and scope of the invention. As such,the breadth and scope of the present invention should not be limited bythe above-described exemplary embodiments, but should be defined only inaccordance with the following claims and equivalents thereof.

What is claimed is:
 1. An underwater acoustic system for estimatingcurrent flow, the system comprising: a stationary multibeam echo sounderin a water column; an acoustic transceiver for communicating with pluraltransducers in a single linear projector array and plural transducers ina single linear hydrophone array; the projector array configuredorthogonally with respect to the hydrophone array to form a Mills crosssuch that multiple projector array transmit fans and multiple hydrophonearray receive beams intersect; the acoustic transceiver for synthesizinga transmitter message and exciting the projector array such thatmultiple fans ensonify the water column; the message including at leastone pulse pair for each fan; the hydrophone array for receivingtransmitter message echoes from scattering centers entrained in thewater column; message echoes time-gated to associate with range cellsalong each beam of each fan; and, a receiver for processing associatedreturn signals via autocorrelation of pulse pairs to calculate a radialvelocity for each range cell of each beam of each fan.
 2. The underwateracoustic system of claim 1 wherein simultaneous consideration of aplurality of the radial velocity estimates provides an estimatedvelocity of the water column at one or more depths.
 3. The underwateracoustic system of claim 2 wherein the radial velocities are Dopplerradial velocities.
 4. The underwater acoustic system of claim 3 wherethe number of fans is greater than or equal to
 2. 5. The underwateracoustic system of claim 4 where the number of beams is greater than orequal to
 8. 6. The underwater acoustic system of claim 2 wherein thetransmitted message includes Barker codes.
 7. The underwater acousticsystem of claim 2 wherein the transmitted message includes orthogonalspread spectrum codes.
 8. The underwater acoustic system of claim 2wherein the multibeam echosounder distinguishes between echoes returnedfrom each of the fans through temporal separation techniques.
 9. Theunderwater acoustic system of claim 2 wherein the multibeam echosounderdistinguishes between echoes returned from each of the fans throughfrequency separation techniques.
 10. The underwater acoustic system ofclaim 2 wherein the multibeam echosounder distinguishes between echoesreturned from each of the fans through code separation techniques. 11.The underwater acoustic system of claim 2 wherein at least one of theradial velocity estimates is estimated via minimization of a costequation.